1 files changed, 161 insertions, 0 deletions
diff --git a/src/cmd/govet/method.go b/src/cmd/govet/method.go
new file mode 100644
index 000000000..55bf11d16
--- /dev/null
+++ b/src/cmd/govet/method.go
@@ -0,0 +1,161 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains the code to check canonical methods.
+
+package main
+
+import (
+	"fmt"
+	"go/ast"
+	"go/printer"
+	"strings"
+)
+
+type MethodSig struct {
+	args    []string
+	results []string
+}
+
+// canonicalMethods lists the input and output types for Go methods
+// that are checked using dynamic interface checks.  Because the
+// checks are dynamic, such methods would not cause a compile error
+// if they have the wrong signature: instead the dynamic check would
+// fail, sometimes mysteriously.  If a method is found with a name listed
+// here but not the input/output types listed here, govet complains.
+//
+// A few of the canonical methods have very common names.
+// For example, a type might implement a Scan method that
+// has nothing to do with fmt.Scanner, but we still want to check
+// the methods that are intended to implement fmt.Scanner.
+// To do that, the arguments that have a + prefix are treated as
+// signals that the canonical meaning is intended: if a Scan
+// method doesn't have a fmt.ScanState as its first argument,
+// we let it go.  But if it does have a fmt.ScanState, then the
+// rest has to match.
+var canonicalMethods = map[string]MethodSig{
+	// "Flush": {{}, {"error"}}, // http.Flusher and jpeg.writer conflict
+	"Format":        {[]string{"=fmt.State", "rune"}, []string{}},            // fmt.Formatter
+	"GobDecode":     {[]string{"[]byte"}, []string{"error"}},                 // gob.GobDecoder
+	"GobEncode":     {[]string{}, []string{"[]byte", "error"}},               // gob.GobEncoder
+	"MarshalJSON":   {[]string{}, []string{"[]byte", "error"}},               // json.Marshaler
+	"MarshalXML":    {[]string{}, []string{"[]byte", "error"}},               // xml.Marshaler
+	"Peek":          {[]string{"=int"}, []string{"[]byte", "error"}},         // image.reader (matching bufio.Reader)
+	"ReadByte":      {[]string{}, []string{"byte", "error"}},                 // io.ByteReader
+	"ReadFrom":      {[]string{"=io.Reader"}, []string{"int64", "error"}},    // io.ReaderFrom
+	"ReadRune":      {[]string{}, []string{"rune", "int", "error"}},          // io.RuneReader
+	"Scan":          {[]string{"=fmt.ScanState", "rune"}, []string{"error"}}, // fmt.Scanner
+	"Seek":          {[]string{"=int64", "int"}, []string{"int64", "error"}}, // io.Seeker
+	"UnmarshalJSON": {[]string{"[]byte"}, []string{"error"}},                 // json.Unmarshaler
+	"UnreadByte":    {[]string{}, []string{"error"}},
+	"UnreadRune":    {[]string{}, []string{"error"}},
+	"WriteByte":     {[]string{"byte"}, []string{"error"}},                // jpeg.writer (matching bufio.Writer)
+	"WriteTo":       {[]string{"=io.Writer"}, []string{"int64", "error"}}, // io.WriterTo
+}
+
+func (f *File) checkCanonicalMethod(id *ast.Ident, t *ast.FuncType) {
+	// Expected input/output.
+	expect, ok := canonicalMethods[id.Name]
+	if !ok {
+		return
+	}
+
+	// Actual input/output
+	args := typeFlatten(t.Params.List)
+	var results []ast.Expr
+	if t.Results != nil {
+		results = typeFlatten(t.Results.List)
+	}
+
+	// Do the =s (if any) all match?
+	if !f.matchParams(expect.args, args, "=") || !f.matchParams(expect.results, results, "=") {
+		return
+	}
+
+	// Everything must match.
+	if !f.matchParams(expect.args, args, "") || !f.matchParams(expect.results, results, "") {
+		expectFmt := id.Name + "(" + argjoin(expect.args) + ")"
+		if len(expect.results) == 1 {
+			expectFmt += " " + argjoin(expect.results)
+		} else if len(expect.results) > 1 {
+			expectFmt += " (" + argjoin(expect.results) + ")"
+		}
+
+		f.b.Reset()
+		if err := printer.Fprint(&f.b, f.fset, t); err != nil {
+			fmt.Fprintf(&f.b, "<%s>", err)
+		}
+		actual := f.b.String()
+		if strings.HasPrefix(actual, "func(") {
+			actual = actual[4:]
+		}
+		actual = id.Name + actual
+
+		f.Warnf(id.Pos(), "method %s should have signature %s", actual, expectFmt)
+	}
+}
+
+func argjoin(x []string) string {
+	y := make([]string, len(x))
+	for i, s := range x {
+		if s[0] == '=' {
+			s = s[1:]
+		}
+		y[i] = s
+	}
+	return strings.Join(y, ", ")
+}
+
+// Turn parameter list into slice of types
+// (in the ast, types are Exprs).
+// Have to handle f(int, bool) and f(x, y, z int)
+// so not a simple 1-to-1 conversion.
+func typeFlatten(l []*ast.Field) []ast.Expr {
+	var t []ast.Expr
+	for _, f := range l {
+		if len(f.Names) == 0 {
+			t = append(t, f.Type)
+			continue
+		}
+		for _ = range f.Names {
+			t = append(t, f.Type)
+		}
+	}
+	return t
+}
+
+// Does each type in expect with the given prefix match the corresponding type in actual?
+func (f *File) matchParams(expect []string, actual []ast.Expr, prefix string) bool {
+	for i, x := range expect {
+		if !strings.HasPrefix(x, prefix) {
+			continue
+		}
+		if i >= len(actual) {
+			return false
+		}
+		if !f.matchParamType(x, actual[i]) {
+			return false
+		}
+	}
+	if prefix == "" && len(actual) > len(expect) {
+		return false
+	}
+	return true
+}
+
+// Does this one type match?
+func (f *File) matchParamType(expect string, actual ast.Expr) bool {
+	if strings.HasPrefix(expect, "=") {
+		expect = expect[1:]
+	}
+	// Strip package name if we're in that package.
+	if n := len(f.file.Name.Name); len(expect) > n && expect[:n] == f.file.Name.Name && expect[n] == '.' {
+		expect = expect[n+1:]
+	}
+
+	// Overkill but easy.
+	f.b.Reset()
+	printer.Fprint(&f.b, f.fset, actual)
+	return f.b.String() == expect
+}